They make a good combination as they occupy different areas of the tank (rays bottom, discus middle) and require the same water chemistry and conditions. They both need slightly acidic and very clean water. You will need to have your filtration system and water management properly organised.
This is especially important with your combo as rays eat a lot, thus polluting the water, and both fish require crystal clear water.
Not a project for a beginner, but well worth it if you can pull it off.
Good luck!
JB